It is a common practice in modern telephone systems to employ multifrequency tones for signaling and data communications. Conventional receivers for such tones require an individual, highly selective narrowband filter for each frequency which can be a component of such a tone. The problem with using narrowband filters is that they tend to be bulky and expensive, even if they are of the time domain or phase-locked loop variety. Also, such filters do not readily lend themselves to large-scale integration because they usually require inductors and/or precise capacitors. Furthermore, since a plurality of such filters is required for a single receiver, the effect of these filters on bulk and price can be substantial.
In receiving tones, two distinguishable operations are performed: tone detection and tone verification. Tone detection, conventionally performed by narrowband filters, is the process of determining that a particular tone is present in a received signal. Tone verification is the process of confirming that a bona fide tone has been received and can include such operations as testing for the presence of the detected tone for some minimum time period and checking that the relative amplitudes of component frequencies are within some predetermined ratio. The instant invention involves tone detection.
It is an object of this invention to perform multifrequency tone detection in a manner that will not require the use of highly selective narrowband filters.
It is another object of this invention to perform multifrequency tone detection in a manner that facilitates manufacture by high-density integrated circuit techniques.